If you take the Happy DayS cast away the only 70s Sitcome Actor Legands are left is Jack Klurgman & Alan Alda everybody else is dead

All in the Family: Carol O' Connor
Brady Bunch: Robert Reed
Mary Tyler Moore: Ted Knight
The Odd Couple: Tony Randall
Three's Company: John Ritter

Mary Tyler Moore is still alive, as is John Travolta, Sherman Hemsley, Gary Coleman, Rob Reiner and many others.